Manufacture of selenium elements such as rectifiers



5 1949 D. DRIVE 2,480,124

MANUFACTURE O ELENIUM EMENTS SUCH AS RECTIFIERS 7 Filed April 6, 1946F/Gl. F/GZ.

I, f 5 \A/ fi 4 8 4 7 2 v x k? v \4 v In Mentor Ado/wig Patented Aug.30, 1949 MANUFACTURE OF SELENIUM ELEMENTS SUCH AS RECTIFIERS DouglasLouis Ashton Driver, London, England, assignor to Standard Telephonesand Cables Limited, London, England, a British company Application April6, 1946, Serial No. 660,292 In Great Britain April 7, 1945 1 Claim.

This invention relates to dry contact rectifiers and photo-electriccells comprising a metal baseplate having thereon a layer ofsemi-conducting material to which is also applied a cunterelectrode.

In order to make satisfactory contact with the counter-electrode, moreparticularly when a number of rectifiers are to be assembled in seriesas a unit, it is necessary to apply a certain amount of pressure. Suchpressure is also advisable when the rectifier is to be protected againstingress of moisture by a coating of varnish as explained in UnitedStates Patent No. 2,314,104, granted March 16, 1943, upon an applicationfiled by E. A. Richards and L. J. Ellison entitled Selenium rectifier.As pointed out in the said patent such pressure causes in the case ofsome rectifiers, such as those using selenium as the semi-conductormaterial, reverse current to be increased. Accordingly it was proposedin said patent to insulate part of the semi-conductor from thecounter-electrode in order to provide a part of the latter upon whichpressure could be exercised without deleterious effect on the rectifier.This could be done by attaching a thin Washer of insulating material tothe semi-conductor before applying the counter-electrode thereto.

It has been found that where the counter-electrode passes over the edgeof the thin insulating washer it tends to develop cracks or to becomeunduly thin so that the resistance in the circuit of the rectifier isunduly increased.

This difficulty is overcome in accordance with the present invention byapplying a thin insulating washer with its upper surface flush with thesurface of the semi-conductor. This may conveniently be done by placingthe insulating washer upon the surface of the semi-conductor when it isin a plastic condition and applying pressure.

The invention will be described as applied to a selenium rectifier. Inknown processes for manufacturing such a rectifier a base plate coatedwith selenium is subjected to heat and pressure as part of the treatmentnecessary to convert the selenium from the vitreous condition to therectiiying allotropic modification. In carrying out the presentinvention a thin paper washer is placed on the surface of the seleniumof such a selenium coated base plate before the heat and pressure isapplied. At the conclusion of the normal heat pressure treatment thepaper washer is found to have its surface flush with the surface of theselenium. A counter-electrode is then applied over the whole of theflush surface.

The rectifier may then be assembled with other rectifiers by means ofpressure exerted on the area of the paper washer.

The invention is applicable also to photo-electric cells, since aspointed out in the copending application, Serial No. 639,469, filedJanuary 5, 1946, by E. A. Richards, entitled Centre contact photo-cell,an insulating washer or layer may be used therein for the same purposeas in a rectifier.

One embodiment of the invention is illustrated in the accompanyingdrawings in which:

Fig. 1 shows'in cross section a base plate coated with selenium and aninsulating washer placed thereon.

Fig. 2 is a cross-section of the base plate of Fig. 1 after having beentreated to produce a rectifier which is assembled with other rectifiersin a unit.

Referring to the drawing a base plate I having a central aperture 2 iscoated with selenium 3,-

as for example by applying selenium to the base plate I when it is at atemperature well above the melting point of selenium, and spreading theselenium over the surface in known manner. The base plate thus coated isthen cooled rapidly.

The coated base plate is then in known manner to be placed in a pressand heat applied, it may be, as described in United States Patent No.2,413,013, issued December 24, 1946, upon an application by Arthur vonHippel entitled Method of making selenium rectifiers, after apreliminary heating without pressure. Immediately before inserting theselenium coated base plate in the press a paper washer 4 having anaperture therein corresponding to the aperture 2 in the base plate I! isplaced on the surface of the selenium 3 as shown in Fig. 1 with theapertures in alignment. The paper washer t may be dished in the centreas shown to facilitate location, the aperture in the washer i being lessthan that in the base plate.

' The excess material of the washer is removed application, Serial No.476,074, filed February 16, 1943, now Patent No. 2,434,960, issuedJanuary 27, 1948, by E. A. Richards, entitled "Cover plate for rectifierdiscs," may then be applied against the counter-electrode to completethe rectifier, which may then be placed on a central spindle I, andassembled with other rectifiers on the spindle into a complete unit, aconducting washer 8 being placed to make contact with the contact plate6 and the base plate of the next rectifier in the assembly. The assemblyis completed by end pressure which, since the washer 8 isnot greater indiameter than the paper washer 4', is exerted in the selenium 3 onlyover the area covered by th paper washer 4.

It is to be understood that the invention is also applicable to thoserectifiers or photo-cells, in which pressure is applied around theperiphery of the disc, the paper washer in this case covering a stripround the periphery. The washer 4 may obviously be made of other solidinsulating material than paper.

Paper washers 4 after they have been applied as described above flushwith the surface of the selenium on a selenium coated base plate, servealso as a protection for the edges of the central aperture 2 in theselenium when the central aperture 2 in the base plate I is formed bypunching out portions of a plate already coated with selenium.

The term selenium cell as used in the following claim, is to beunderstood as embracing both selenium photo-cells and seleniumrectifiers.

What is claimed is:

In a process for the manufacture of a stratified selenium cell having alocalized mass of solid electrically insulative material in the seleniumlayer wherein a base element is provided with an overlying seleniumlayer which is hot-pressed to convert the selenium to the desiredsemiconductive allotroplc modification, the improvement that comprisesplacing the solid electrically insulative mass upon the selenium layerprior to the hotpressing, and thereafter hot-pressing against a fiatplaten whereby the insulative mass is pressed into the selenium layerflush with the surface thereof and rigidly consolidated therewith,simultaneously with conversion of the selenium to the desired allotropicmodification.

DOUGLAS LOUIS ASHTON DRIVER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,314,104 Richards Mar. 16, 19432,345,122 Herrmann Mar. 28, 1944

